Asteroids and comets (and small moons) can be colonized inside by digging them out and leaving a protective shell. In asteroid comets relatively close to a star, light let in through windows could be directed toward habitats and greenhouses by mirrors or prisms that reflect light but not dangerous radiation. In comet colonies, energy could be produced by fusion reactors or possibly by vacuum energy. Dug out colonies would be a practical way to colonize small moons in radiation belts because the empty shell, several metric meters thick, would shield out the radiation. Another possibility is artificial gravity colonies. See also static vapor air retention.